For effective use of direct sequence CDMA systems for digital mobile cellular telephone and personal communication network applications, a detection technique must be used which performs well at low signal to interference ratios. Coherent detection is preferred to non-coherent detection because it has better performance in the slow fading environments which typify personal communication channels. To apply coherent detection, the channel impulse response at a receiver must be known, and this can be achieved by transmitting pilot symbols.
Pilot symbols can be transmitted in two ways; a) a dedicated pilot channel, i.e. one pilot channel for each user, in which pilot symbols are embedded periodically (time- or code-multiplexed) in the same channel as the data symbols, or b) a common pilot channel, i.e. one pilot channel for all users, in which pilot symbols are continuously sent on a separate channel in parallel with data channels.
An advantage of dedicated pilot channels is that power can be varied, so that a mobile at a boundary of a cell can ramp up the power of its received symbols to overcome channel propagation as well as fast fading; however the system relies on good  statistical multiplexing of users to ensure that there is always spare transmitter capacity to meet a sudden demand from a mobile for increased power, which can create instability.
The well known differences between the two arrangements will now be described with reference to FIGS. 1-6.
FIG. 1a shows the sector coverage angle alpha (e.g. 30° to 40°) over which a small base station transmits. FIG. 1b indicates by the enclosed area the energy Ed required to transmit data, and this is assumed to be constant. FIG. 1c indicates by the shaded area the energy Ep required to transmit pilot symbols in either a dedicated pilot channel or a common pilot channel.
FIG. 2 illustrates energy requirements in a common pilot channel arrangement, and is effectively a merger of FIGS. 1b and 1c; a single continuous pilot channel is broadcast to all users.
FIG. 3 illustrates energy requirements in a dedicated pilot channel arrangement; each of the N users (where N±5) has a different energy requirement E1 to E5, shown by the shaded and crosshatched areas. The total energy requirement for the pilot channels is N*Ep. This arrangement assumes there is no power control facility to vary power transmission.
FIG. 4 shows a variation of FIG. 3 including a power control facility. The power supplied to each pilot channel can be controlled individually, as indicated by the different areas of the shaded and crosshatched bands E6 to E10. At certain times, in theory, the pilot in a channel can even be switched off completely, saving energy, and allowing other data or control information to be transported by that channel. Pilot energy requirement is ΣEp,i·βi where βi is the scaling factor for each user, dependant on power control and time multiplexing. β is between 1 and 0, i.e. it is small when a mobile is close to its base station.
However comparison with FIG. 1c shows that the total power used is unchanged.
FIG. 5 indicates energy requirements where spatially adaptive antennas are used. Data energy is transmitted in much narrower sections α1 to α4 within the sector angle α, i.e. a beam forming technique is used. The narrow sectors α1 to α4 are directed towards active mobiles, and the pilot energy required for each narrow sector is also varied in accordance with need, as indicated by the shaded areas. The total energy requirement is greatly reduced. The pilot energy requirement is
      ∑    i    N    ⁢          ⁢            β      i        ⁢          E      p        ⁢          G      A      where GA is the gain of the directed antennas.
FIG. 6 shows that, in addition to the directed channels of FIG. 5, some common channel facility is required across the whole sector angle α, e.g. for mobiles attempting to make a call, and the data power for this is indicated at Edc, between the directed sectors with the associated pilot energy indicated by the cross-hatched areas Epc. Pilot energy requirements are
                    ∑        i        N            ⁢                        β          i                ⁢                  E          p                ⁢                  G          A                      +                  ∑        i        C            ⁢                        β          i                ⁢                  E          p                      ,where C is the number of common channels.